Elastomeric compositions in which butyl rubber is dynamically vulcanized in the presence of polyolefins such as polypropylene have resulted in thermoplastic compositions which have been proposed for use for most applications where thermoplastic elastomers would be of advantage. However, many of the products produced by the dynamic vulcanization of butyl rubber in the presence of materials such as polypropylene have not resulted in products which are suitable for vial stoppers, blood tube stoppers and dropper bulb applications, for example, in the pharmaceutical industry. Because of extremely high quality control and concern for the integrity of the product contained in pharmaceutical containers, thermoplastic elastomers have been slow to be accepted as candidates for container closures, vial stoppers, dropper bulbs and the like. The product must have mechanical strength and set resistance properties, but also must provide moisture and oxygen barriers while not allowing undesirable materials to be extracted from the product. Until the present invention, such a material has not been developed which has all of the required properties for use successfully in the pharmaceutical industry.
U.S. Pat. No. 4,130,534 describes a thermoplastic composition containing polyolefin resins and crosslinked butyl rubber. The product is formed by dynamic vulcanization, in which the rubber is the continuous phase prior to crosslinking but become after the crosslinking step.
U.S. Pat. No. 4,501,842 describes an adhesive in which a halogenated butyl rubber is combined with a styrene/ethylene butylene/styrene block thermoplastic polymer (SEBS) and other materials to form an effective adhesive. Zinc oxide is used as a strength increaser.
U.S. Pat. No. 4,593,062 describes the combination of polyolefins, halogenated butyl rubber and polychloroprene in which these rubbers are dynamically vulcanized in the presence of the polyolefin. This patent contains an extensive description of the dynamic vulcanization process.
U.S. Pat. No. 4,607,074 combines polypropylene, a zinc oxide cured halogenated butyl rubber, and a second rubber, preferably EPDM. Various curing agents are suggested for curing the rubber during dynamic vulcanization, including accelerators which assist the zinc oxide cure.
U.S. Pat. No. 4,616,052 describes elastomeric compositions which are used as stoppers and syringe plunger tips. The principal component of the product is a dynamically vulcanized ethylene-propylene-diene terpolymer and polypropylene mixed with butyl rubber in the range of perhaps 5 to about 20 percent. A hindered amine is present to act as a stabilizer.
U.S. Pat. No. 4,616,064 describes a mixture of polyolefins, SEBS polymers and materials such a polysiloxanes.
One particular method for preparing halogenated butyl rubbers through isomerization is described in U.S. Pat. No. 4,634,741. All of the cure processes for these butyl rubbers are based upon a zinc oxide cure system.
U.S. Pat. No. 4,639,487 describes a different system using a butyl rubber mixed into an ethylene copolymer resin which is dynamically vulcanized. The preferred curing system contains zinc oxide.
A preferred method and product is described in a commonly owned copending application titled HALOBUTYL THERMOPLASTIC ELASTOMER, now U.S. Pat. No. 4,810,752. In this patent, thermoplastic elastomer compositions are provided which are suitable for use as a vial stopper, blood tube stopper, dropper bulb or syringe tip in the pharmaceutical industry and other applications where low hardness and a good barrier to moisture and oxygen is needed.
Other polymers have also been considered for various applications in the pharmaceutical industry and elsewhere. One class of compounds which have not found application in the pharmaceutical industry are those generally described as silicone modified EPDM. These materials are described in U.S. Pat. No. 4,150,010, U.S. Pat. No. 4,201,698, and U.S. Pat. No. 4,365,042. In each of these three patents, polyorganosiloxanes are used to modify EPDM type polymers to improve the oil and solvent resistance of the latter EPDM elastomers. These products are cured using conventional crosslinking techniques, either alone or with fillers and other additives. Products are sold under the ROYALTHERM trademark, such as ROYALTHERM 1421, by the Uniroyal Chemical Company, Inc. and are representative of the products described in the three patents just referred to. The silicone modified or grafted EPDM has improved heat resistance and mechanical properties, and can be vulcanized with either a peroxide system or a sulfur/accelerator system.
Other patents relating to EPDM modified with silicone include U.S. Pat. No. 4,365,038; U.S. Pat. No. 4,234,702; and U.S. Pat. No. 4,341,675.
None of these silicone modified EPDM materials have been suggested for use as a replacement for butyl rubber or other products which are used in the pharmaceutical industry. In actual fact, none of these materials are capable of being formulated into commercially acceptable vial stoppers, etc.